1. Field of the Invention
The present teaching relates to a kneading apparatus for producing a thermoplastic resin molded product by using a pressurized fluid, a method for producing the thermoplastic resin molded product based on the use of the kneading apparatus, and a foam injection molding method.
2. Description of the Related Art
In recent years, various types of injection molding methods and extrusion molding methods have been investigated, in which any pressurized fluid is used. Examples of the pressurized fluid include pressurized carbon dioxide and pressurized nitrogen. In the case of the molding method as described above, a fluid having an extremely high pressure is introduced into a molten resin, and hence it is possible to produce molded products having various functions. For example, in order to mutually compatibly dissolve polymers which are incompatible with each other, an injection molding method and an extrusion molding method are suggested for a polymer alloy, wherein molten resins and high pressure carbon dioxide are brought in contact and kneaded with each other in a plasticizing cylinder (Japanese Patent Application Laid-open No. 2003-94477 and Proceedings of 17th JSPP Symposium of Japan Society of Polymer Processing, 227 (2009)). In the molding methods as described above, the molten resin and pressurized carbon dioxide are brought in contact and kneaded with each other by means of a kneading apparatus which is provided with a screw contained in a plasticizing cylinder.
On the other hand, a molding method is suggested, wherein supercritical carbon dioxide is introduced into a molten resin at an intermediate position of an extrusion machine having a vent portion in order to remove any hardly volatile component from the thermoplastic resin (Japanese Patent Application Laid-open No. 11-292921). Further, an injection molding method is suggested, wherein a molten resin of thermoplastic resin is injected and charged into a mold from a plasticizing cylinder, and then a pressurized fluid, which contains supercritical carbon dioxide and a functional material such as an organic metal complex or the like, is introduced into the mold to thereby produce a thermoplastic resin molded product including the functional material dispersed on a surface (Japanese Patent No. 3964447 which corresponds to US2003/228485).
In the meantime, the solubility of pressurized carbon dioxide with respect to the resin is low. Therefore, in the case of the molding method including the step of bringing the molten resin and the pressurized carbon dioxide in contact with each other as described above, it is difficult to allow a large amount of pressurized carbon dioxide and the molten resin to be brought in contact and kneaded with each other. Therefore, when the functional material is used together with pressurized carbon dioxide, it is also difficult to introduce the functional material into the molten resin at a high concentration. From such a viewpoint, a method for producing a molded product is suggested, wherein an kneading apparatus, in which an introducing port for introducing pressurized carbon dioxide is provided on an upper side surface of a plasticizing cylinder and a vent is provided on the downstream side from the introducing port, is used so that a molten resin, pressurized carbon dioxide, and a functional material are brought in contact and kneaded with each other in the plasticizing cylinder, and then the resin internal pressure of the molten resin is lowered before being injected and charged into a mold to separate only gasified carbon dioxide from the molten resin and discharge carbon dioxide from the vent (Japanese Patent Application Laid-open No. 2009-298838 which corresponds to US2011/104380). According to this molding method, it is possible to improve the concentration of the functional material to be introduced into the molten resin, while controlling the concentration of pressurized carbon dioxide in the molten resin.
Further, in recent years, a foam injection molding method, which is based on the use of a physical foaming agent such as nitrogen and carbon dioxide in a supercritical state as the pressurized fluid, is researched and practically used (Japanese Patent No. 2625576 which corresponds to WO92/17533, Japanese Patent No. 3788750 and Japanese Patent No. 4144916 which corresponds to U.S. Pat. No. 5,997,781). According to Patent Documents 5 to 7, the physical foaming agent is introduced into a hermetically closed plasticizing cylinder, and the physical foaming agent is brought in contact with and dispersed in the plasticized and melted resin. The molten resin, in which the foaming agent is dispersed, is weighed while maintaining the high pressure in the plasticizing cylinder to such an extent that the physical foaming agent is in the supercritical state, and the molten resin is injected and charged into a mold. The supercritical fluid, which has been compatibly dissolved in the molten resin upon the injection and charging, is subjected to sudden pressure reduction and gasified. The molten resin is solidified, and thus foams (bubbles) are formed at the inside of the molded product.
The foam injection molding, which uses the supercritical fluid, is advantageous in that the process is clean, any residue of the foaming agent does not remain, and the mechanical strength of the molded product is hardly lowered because the foamed cell diameters become fine and minute, as compared with the foam injection molding which uses any chemical foaming agent. Further, the high pressure physical foaming agent functions as a plasticizer for the molten resin. Therefore, the following advantages are also provided. That is, the viscosity of the resin is lowered upon the injection and charging, and the fluidity is improved. Sink marks are suppressed, which would be otherwise caused by the shrinkage when the resin is solidified on account of the gas pressure upon the foaming. The latent heat is deprived from the interior of the molten resin upon the foaming, and thus the cooling strain and the warpage are decreased.
Further, the supercritical fluid has a high density, and the supercritical fluid is weighed with ease. Therefore, the supercritical fluid is advantageous to stabilize the amount of introduction into the molten resin. For example, a method of Japanese Patent No. 3788750 has been disclosed as a technique for stably and quantitatively supplying the physical foaming agent as described above into the plasticizing cylinder. According to Japanese Patent No. 3788750, the resin internal pressure is controlled by the pressure of a load cell connected to a screw, i.e., by the back pressure of the screw, and thus the supply amount of the foaming agent is controlled. Japanese Patent No. 4144916 discloses a system wherein the back pressure of a screw is raised, and the pressure of the forward end of the screw at which a physical foaming agent is dissolved is maintained at a pressure in the supercritical state so that the separation is suppressed between the resin and the physical foaming agent.
In the meantime, in the production method in which the modifying material dissolved in pressurized carbon dioxide is kneaded with the molten resin in the plasticizing cylinder as described above and only carbon dioxide is further discharged, a problem arises such that the amount of pressurized carbon dioxide, which is actually introduced, varies or fluctuates in every shot, and the amount of pressurized carbon dioxide cannot be controlled.
Further, in the production method in which carbon dioxide is discharged from the plasticizing cylinder, a problem of vent up arises. The vent up is such a phenomenon that the molten resin simultaneously leaks from a discharge port for discharging pressure-reduced carbon dioxide. The main cause of the bent up is considered as follows. That is, the resin viscosity is lowered due to the kneading of carbon dioxide, and the resin undergoes the volume expansion due to the sudden pressure reduction to be performed for the purpose of gas discharge.
Further, a problem as described below arises in the foam injection molding based on the use of the physical foaming agent described above. In order to raise the forming density of a molded product in the foam injection molding, it is effective that the physical foaming agent is dissolved in the molten resin at a concentration approximate to the saturated solubility (saturated concentration) in the plasticizing cylinder. Accordingly, the physical foaming agent can be supersaturated with respect to the molten resin in a high pressure region upon the injection and charging into the mold, and it is possible to generate a large number of foaming nuclei.
In the case of the conventional foam injection molding method, the molten resin is extruded to the frontward of the screw in accordance with the plasticization, and the pressure of the molten resin is controlled by the back pressure of the screw. However, the back pressure and the pressure are different from each other in many cases at the frontward portion of the screw at which any difference tends to arise in the viscosity and the density of the resin on account of the fact that the foaming agent is not kneaded uniformly or homogeneously, wherein it has been impossible to sufficiently control the pressure. Therefore, the molten resin and the physical foaming agent tend to cause the phase separation at the frontward portion of the screw. In particular, this phenomenon is conspicuous when the compatibility is low between the both. Further, in the plasticizing step based on the rotation of the screw, the physical foaming agent is kneaded by utilizing the shearing of the screw in a short period of time. Therefore, it has been difficult to dissolve the physical foaming agent in an amount approximate to the saturated solubility in the molten resin. Further, if the concentration of the physical foaming agent in the molten resin is high, the risk is further raised to cause the phase separation between the molten resin and the physical foaming agent. Therefore, in the case of the conventional foam injection molding method, it has been necessary that the concentration of the physical foaming agent should be lowered to be about ⅕ to 1/10 of the saturated solubility.
It has been necessary that the pressure of the physical foaming agent introduced into the plasticizing cylinder should be raised in order to form a large number of foaming nuclei during the injection and charging into the mold while providing the low ratio of the concentration of the physical foaming agent in the molten resin with respect to the saturated solubility as described above. However, this results in a factor to raise the initial cost of the apparatus.
The conventional foam injection molding method involves another problem such that the amount of the foaming agent introduced into the molten resin varies or fluctuates in every shot. The following artifice has been made to quantitatively supply an amount of the foaming agent introduced into the molten resin in the conventional foam injection molding method. That is, for example, the pressure in the plasticizing cylinder, which is provided in the vicinity of the introducing port for the physical foaming agent, is subjected to the feedback, and the amount of introduction of the physical foaming agent is determined so that the differential pressure, which is provided between the pressure subjected to the feedback and the pressure of the physical foaming agent to be introduced, is constant.
However, the pressure in the plasticizing cylinder subjected to the feedback is the pressure provided one shot before, wherein the feedback is not perform in real-time. Further, the pressure in the plasticizing cylinder subjected to the feedback is obtained by detecting the back pressure of the screw. The amount of dissolution of the foaming agent is not completely uniform in the plasticizing cylinder. Therefore, the pressure in the plasticizing cylinder is sometimes distributed while providing the pressures different from the screw back pressure to be detected. Therefore, the amount of the foaming agent actually introduced into the molten resin is varied or fluctuated for every shot, and it has been impossible to control the amount of the foaming agent.
The present teaching relates to a kneading apparatus in which a pressurized fluid is kneaded with a molten resin in a plasticizing cylinder and at least a part of the pressurized fluid is discharged and a production method for producing a thermoplastic resin molded product in which the kneading apparatus is used. An object of the present teaching is to constantly stabilize the amount of the pressurized fluid to be introduced for each shot. Another object of the present teaching is to suppress the vent up of the molten resin in the kneading apparatus and the production method. Further, the present teaching relates to a foam injection molding method for a thermoplastic resin. Still another object of the present teaching is to dissolve a physical foaming agent in a plasticizing cylinder, in a molten resin at a concentration approximate to a saturated solubility before injection.